U.S. Pat. No. 6,286,959 B1 (the “'959 Patent”), assigned to the assignee of this application, discloses wavefront sensing using a distorted grating to determine the characteristics of a wavefront that has passed through a cornea (either in vitro or in vivo). More specifically, this patent relates to the use of wavefront sensing using a distorted grating to identify corneas that have been surgically modified. The apparatus includes a distorted grating and an imaging lens which have a pupil plane, first and second virtual planes and an image plane.
With reference to FIG. 1 of the '959 Patent, apparatus 11 for determining the characteristics of a wavefront includes a source of light 13, a distorted grating (sometimes referred to as a “distorted diffraction grating”) 17, a high quality imaging lens (or lens set) 19, and a detector 21 (either film or electronic) having a detector plane 23. Grating 17, lens 19 and detector 21 are sometimes referred to as wavefront sensor 24. Apparatus 11 also includes a beam path 25, a pupil plane 27, first virtual plane 29, second virtual plane 31, and a data processor 33. Data processor 33, connected to detector 21 via a data acquisition device such as a frame grabber (not shown), stores the images from detector 21 and determines the wavefront from the stored images. The representation of the virtual planes between source 13 and sensor 24 is for convenience only.
With grating 17 in close proximity to lens 19 (typically these two elements would, in fact, be in contact with each other along beam path 25), the 0, +1 and −1 diffraction orders of grating 17, image pupil plane 27, virtual object plane 29 and virtual object plane 31 are projected onto detector plane 23. The higher order diffraction orders are cut off by an appropriately placed field stop (not shown) so as not to contaminate the image of the 0 and +1 and −1 orders. Further, with the zero order being an image of the pupil plane 27, the images in the +1 and −1 diffraction orders correspond to virtual image planes equidistant from and an opposite sides of pupil plane 27. The grating is distorted according to,
            Δ      x        ⁡          (              x        ,        y            )        ≂                              W          20                ⁢        d                    λ        ⁢                                  ⁢                  R          2                      ⁢          (                        x          2                +                  y          2                    )      where λ is the optical wavelength, x and y are Cartesian co-ordinates with an origin on the optical axis and R is the radius of the grating aperture which is centered on the optical axis. The parameter W20, defines the defocusing power of the grating, and is the standard coefficient of the defocus equivalent on the extra pathlength introduced at the edge of the aperture, in this case for the wavefront diffracted into the +1 order. The phase change (Øm) imposed on the wavefront diffracted into each order m is given by,
            φ      m        ⁡          (              x        ,        y            )        =      m    ⁢                  2        ⁢        π        ⁢                                  ⁢                  W          20                            λ        ⁢                                  ⁢                  R          2                      ⁢          (                        x          2                +                  y          2                    )      
With nothing in pupil plane 27 of apparatus 11 (e.g., cornea container 15 removed) and source 13 present, the images recorded on detector plane 23 are as illustrated in FIG. 1B of the '959 Patent.
While the '959 Patent discloses a sensor, it does not disclose any method or apparatus for correcting the wavefront of a beam.
U.S. Pat. No. 7,232,999 B1 (the “999 Patent”) and pending U.S. patent application Ser. No. 11/820,651 filed Jun. 19, 2007 (which claims the priority of the '999 Patent), both assigned to the assignee of this application and incorporated by reference, disclose the use of distorted grating based wavefront sensors to measure wavefronts of radiation. More particularly, in the preferred embodiment the invention disclosed therein involves positioning a beam of light containing the wavefront to be characterized onto a distorted grating, using the grating to produce a plurality of images, determining the infrared wavefront from the plurality of images and analyzing the wavefront for features that characterize the infrared wavefront.
With reference to FIG. 7 of the '999 Patent wavefront 101 to be measured is directed onto pupil plane 103, the wavefront at pupil plane 103 is then redirected onto grating 105, modifying wavefront 101, which modified wavefront is subsequently focused onto detector 107 through lens 109. Grating 105, lens 109 and detector 107 constitute the wavefront sensor. Optional pupil relay and magnification optics 115 can be used to orient and resize wavefront 101 as required by the application being used.
With reference to FIG. 8 of the '999 Patent, laser 121 generates light beam 123 which is passed through attenuator 125 and is re-directed using optics 127 and 129. Mirrors 131 and 133 are used to disperse and re-collimate beam 123 which is then directed through aperture 135 (collimation is not required). It is the wavefront as it exists at aperture 135 that will ultimately be imaged onto detector 145. Beam 123 is subsequently directed through lenses 137 and 139 which are used to position and magnify beam 123. Beam 123 is then passed through diffraction grating 141 before being focused by lens 143 onto the focal plane of detector (infrared camera) 145. Lens 143 serves to focus the beam 123 as modified by grating 141 onto a detector 145. Grating 141, lens 143 and detector 145 constitute the wavefront sensor.
The '999 Patent does not, however, disclose either a method or apparatus for: (1) correcting the output beam of a laser; or (2) correcting the image of an object prior to detecting such image.
Some of the most widely known work in the field of adaptive optics has been done for astronomical purposes; attempting to correct atmospheric turbulence to allow telescopes at low altitudes to perform as well as high-altitude telescopes (e.g. Mauna Kea at 14,000 feet elevation) or, better yet, like space based telescopes. Most adaptive optics systems use a Shack-Hartmann wavefront sensor, which requires a point source as its reference as, basically, a Shack-Hartmann sensor calculates the centroid of an image of the reference, which requires that reference is small and well defined. Alternatively, some astronomical adaptive optics systems use a wavefront sensing technique called phase diversity, which takes two defocused images of the reference and so, again, relies on the reference being small and well defined.
Ideally, in order to correct the image, wavefront measurements are made on a perfect source that has propagated along the same path as the image. This way correcting the wavefront of the ideal source simultaneously corrects imaging through optics looking along the same path. The problem is finding a perfect wavefront source to measure. For astronomy, a star can be used as the reference (as it is small and well defined). However, it also has to be bright which severely limits the regions of the sky that can be observed (astronomical telescopes typically have a very small field of view and so the chances of a bright star being within the field of view of the object which an astronomer wants to observe is very small). A solution to this problem is to use a laser to create a bright virtual star (an ideal reference) by exciting sodium atoms in the upper atmosphere. This artificially generated reference is typically called a “guide star”.
This same basic technique (projecting a laser beam through the optical system, measuring and correcting the return beam and, hence, correcting the imaging performance of the system) has been adapted to other, non-astronomical applications (e.g. enhanced retinal imaging). U.S. Pat. No. 6,331,059 B1 (the “'059 Patent”), discloses an improved fundus retinal imaging system in which a conventional fundus retinal imager is combined with a multispectral source, a dithered reference, a wavefront sensor, a deformable mirror and a high resolution camera. More specifically, the '059 Patent discloses an ophthalmic instrument having a wide field of view (up to 20 degrees) including a retinal imager, (which includes optics for illuminating and imaging the retina of the eye); apparatus for generating a reference beam coupled to the imager optics for measuring the wavefront produced by optical aberrations within the eye and the imager optics; wavefront compensation optics coupled to the imager optics for correcting large, low order aberrations in the wavefront; a high resolution detector optically coupled to the imager optics and the wavefront compensation optics; and a computer (which is connected to the wavefront sensor, the wavefront compensation optics, and the high resolution camera), including an algorithm for correcting small, high order aberrations on the wavefront and residual low order aberrations. The wavefront sensor includes a Shack-Hartmann wavefront sensor having a lenslet array and a detector positioned in the front surface of the lenslet array for producing a Hartmannogram. See, generally, FIG. 1 of this reference.
U.S. Pat. No. 6,736,507 B2 (the “'507 Patent), which is a continuation-in-part of the '059 Patent, discloses the use of a distorted grating wavefront sensor as an alternative to the Shack-Hartmann wavefront sensor. See, col. 3, ll. 18-21 and col. 5, ll. 15-27. However, regardless of which sensor is used, all the other optics and electronics remain the same. Also, overall the methodology remains unchanged. Thus, for instance, the apparatus for generating a reference beam coupled to the image optics to form a reference area on the retina is used with both the Shack-Hartmann wavefront sensor and the distorted grating sensor.
All of the foregoing adaptive optics systems, including the systems described in the '059 and '507 Patents, include wavefront characterization and correction. Further, all include the following steps (and the associated apparatus for accomplishing such steps): (a) acquiring data from the wavefront to be characterized and corrected; (b) using the acquired data to mathematically reconstruct the wavefront; (c) from the reconstructed wavefront computing either the slope or the curvature of such wavefront (depending on what type of data is needed to drive the mirror used to correct the wavefront); (d) using the slope (or curvature) data to generate signals to drive the mirror; and (e) driving the mirror to correct the wavefront. In the case of adaptive optics imaging systems, an artificially generated reference (e.g., guide star) is also necessary, in which case step (a) becomes: acquiring data from the wavefront of the artificially generated reference.
The foregoing, without the artificially generated reference, is schematically illustrated in FIG. 1, in which adaptive optics system 11 includes wavefront modulator 13, wavefront sensor 15, data acquisition device 17, prior art processor 19 and amplifier 21. Wavefront modulator 13 would, typically, be a deformable mirror (which includes actuators); sensor 15, a Shack-Hartman sensor. As illustrated, data acquisition device 17 includes a detector 23 (e.g., CCD (Charged Coupled Device) or CMOS (Complementary Metal Oxide Semi-conductor)) and a mechanism (e.g., routine or hardware) 25 for digitizing the images captured by detector 23. Processor 19 includes computer routine 27 for processing the raw digital data from converter 25 into information utilized by the analysis technique associated with the data acquisition device 17 (e.g., a Shack-Hartman sensor or a distorted grating wavefront sensor). Processor 19 also includes a routine 29 for mathematically recreating the detected wavefront, and a routine 31 for calculating the slope or the curvature of the created wavefront (sometimes referred to as wavefront modulator commands), depending on what type of data is needed to drive wavefront modulator 13. Finally, processor 19 includes a routine 33 for converting the digital information from routine 31 to analog. Amplifier 21 provides the power to drive the actuators of deformable mirror 13. Variations of the foregoing include incorporating detector 23 into wavefront sensor 15. Converter 25 can be part of data acquisition device 17, incorporated into processor 19 or a stand alone device. Amplifier 21 can be a separate device as illustrated, or combined with wavefront modulator 13.
In addition to a deformable mirror, adaptive optics systems may include a mechanism of correcting the tip/tilt of the beam (sometimes also referred to as removing jitter). With reference to FIG. 2, tip/tilt correction system 41 includes tip/tilt mirror 43, beam splitter 45 and tip/tilt sensor 47 (e.g., a quad cell or a position sensitive detector (or PSD)). The adaptive optics system also includes wavefront modulator 49, beam splitter 51 and wavefront sensor 53. Beam splitter 45 divides beam 55 into portion 57 which is directed to sensor 47 and portion 59 which is directed onto wavefront modulator 49. Similarly, beam splitter 51 divides beam portion 59 into two portions, 61 and 63, the latter of which is directed to wavefront sensor 53. Wavefront sensor 53, via processor 19 (shown only in FIG. 1), controls wavefront modulator 49 in the manner described above with regard to FIG. 1. However, as is evident from FIG. 2, the control loop for tip/tilt mirror 43 is, and has to be, separate from the control loop for wavefront modulator 49.
The foregoing adaptive optics systems have the following disadvantages: (a) they require a routine for mathematically reconstructing the measured wavefront, which routine is computationally intensive; (b) they require a routine for determining the slope (or the curvature) of the reconstructed wavefront, which routine is also computationally intensive; (c) when used for imaging they require the use of an artificially generated reference (e.g., a guide star); and (d) they require a separate data collection/control loop for a tip/tilt correction device.